Apparatus and method for coupling laser energy into small core fibers

ABSTRACT

A fiber optic connector for coupling focused radiant energy from a laser to a fiber optic conductor includes a secondary transmission path and one or more reflectors and/or heat sinks for deflecting errant radiant energy to a location where it be safely dissipated. In addition, coupling of the radiant energy to the cladding of the fiber is minimized by stripping or at least partially removing the cladding to reduce the amount of cladding in the area that extends to the focal plane of the radiant energy source.

This application is a divisional of U.S. patent application Ser. No.10/370,453, filed Feb. 24, 2003, which claims the benefit of provisionalapplication Nos. 60/358,309, filed Feb. 22, 2002, and 60/385,890, filedJun. 6, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a method and apparatus for couplingradiant energy from a radiant energy source to the conducting medium ofa primary optical system. The primary optical system may be, but is notnecessarily limited to, an optical fiber system.

The invention also relates, in applications where the primary opticalsystem is a fiber optic conductor, to a fiber optic connector forcoupling focused radiant energy from a laser to a fiber optic conductor,and to a method of coupling radiant energy from the laser to theconductor, in which radiant energy that fails to couple with theconductor is directed away from sensitive structures, and preferably(though not necessarily) deflected back to the laser system itself, bymeans of a secondary transmission path and one or more reflectors and/orheat sinks, so that the errant radiant energy can be safely dissipated.

By directing excess radiant energy in this manner, the excess radiantenergy is prevented from damaging the connector and optical componentsin the connector, thereby enabling the use of smaller fibers withgreater coupling tolerances, and more specifically permitting the use ofoptical fibers having core diameters smaller than the focused spot ofthe laser source, that are not precisely aligned with the focused spot,and/or that have an acceptance angle too small to accept all of thefocused radiant energy.

Finally, the invention also relates to a fiber optic termination, and toa method of terminating an optical fiber, that reduces or eliminateslaunching energy into the cladding of the fiber.

2. Description of Related Art

The invention provides a solution to the problem of errant radiantenergy when coupling radiant energy from lasers or other coherentradiant energy sources, which are typically but not necessarilymonochromatic, to relatively small primary optical transmission systems,for example, optical fibers used in medical devices such as scalpels orlithotripter fibers. Such optical fibers are especially useful toimplement recently developed, minimally invasive surgical techniques.

An example of an apparatus to which the principles of the invention maybe applied is illustrated in FIGS. 1 and 2. The apparatus includes alaser system 12 and a standard connector coupler 18 for coupling aconnector such as connector 30 shown in FIG. 2, which couples the outputof the laser system to a primary optical system such as an optical fiberor fiber cable 32. The laser may be a high energy pulse or continuouswave laser that generates a monochromatic radiant energy output beam 17.For example, the laser system may be a Holmium:YAG laser that generatesan output formed of pulses on the order of 250μ seconds in pulse widthand energy levels ranging up to 1800 mj/pulse with an average power of12 Watts. The output beam 17 is passed through a condensing lens to forman output beam 17 a that is focused on a spot 17 b in the vicinity ofinput focal plane 16 and centered in a connector coupler 18 mounted onthe laser enclosure 15 using respective X, Y, and Z adjustments 12 a–12c. When connector 30 shown in FIG. 2 is secured to connector coupler 18by locking member 40 (which may, for example, be an internally threadednut), the connector ferrule 31 is ideally centered on focused spot 17 band the distal end of the connector ferrule is at the focal plane 16. Inthis example, the focused spot size at the focal plane 16 is on theorder of 365 microns and the relative power density at the focal planefor a 365 micron spot with an average power of 12 watts is approximately11.5 kW/cm². On the other hand, the power density 6 mm beyond the focalplane 16 is reduced by a factor of 50.

Ferrule 31 of connector 30 is typically a metal elongated hollow bodymember into which is inserted the optical fiber or fiber cable 32. Theproximal end has a fiber clearance hole 38 drilled close to the outsidediameter of the fiber 35. To secure the fiber 35 to the ferrule 31, asmall portion of the fiber optic cable 32 is stripper away exposing theglass fiber 35. Before the stripped fiber is placed inside ferrule 31,an adhesive 34 may be applied to a small portion of exposed fiber 35 andthe exposed fiber is passed through the internal diameter of the ferruleto its distal end. The extreme distal portion of the exposed fiber exitsthe ferrule through fiber clearance hole 38. Later, after adhesive 34 iscured, the exposed fiber 35 is trimmed and polished such that the distalend of the ferrule and the distal end of the fiber 35 are flush.Alternately, the fiber 35 may be secured within the ferrule by crimpinga portion of the cable 32 to the ferrule using a sleeve.

The errant radiant energy problem arises when the primary opticaltransmission system approaches or is smaller than the size of thefocused beam of radiant energy. For example, the smaller the diameter ofan optical fiber, the more difficult it is to focus energy from thelaser into the core. If the core diameter is smaller than that of thefocused spot of the laser source, or if the focused radiant energy tothe core is misaligned or greater than the fiber's acceptance angle,then energy will be transferred to structures that make up the coupleror that surround the core. The density is often great enough to soften,melt, or fuse any materials which are not highly optically transmissiveor reflective. In many cases the energy density can be so great thatphoto thermal ablation may occur in the metal housing of the connectorthat couples the laser to the fiber, causing the metal to explosivelyform a plume mixture gases and micron size particles, which re-depositand contaminate the focusing lens. Further lasing into the contaminationcan create extreme localized heating which ultimately destroys thefocusing lens.

There is therefore a need for a coupling apparatus and method thatminimizes the impact of radiant energy that fails to couple to the coreof the optical fiber (or other primary optical transmission system).

Furthermore, the coupling apparatus and method must be compatible withexisting laser systems and connectors, such as the one illustrated inFIG. 1. For example, in medical applications, connectors that implementthe invention generally should be compatible with standard medical laserindustry connectors, such as the SMA 905 standard connector.

Finally, even when all of the energy that fails to couple to the fiberis dissipated, a further problem arises in that some of the energy thatcouples to the fiber will couple to the cladding of the fiber ratherthan to the core, resulting in the problem that the cladding will act asa secondary wave guide and leak energy into surrounding coating duringtight bends, such as my occur when the optical fiber is used for laserlithotripsy after it has been passed through the working channel of anendoscope. While the amount of coupling may be reduced by tapering, thecore and cladding may mix, causing light to also mix into the cladding,and higher order modes may be created which are more subject to lossduring a bend than lower order modes. By way of background, it wasproposed in U.S. Pat. No. 6,282,349 to fuse the cladding to the ferrulein which it is placed, but the cladding fusion scheme described in thispatent did not involve removal of some or all of the cladding at the endof the fiber reduce coupling of laser energy into the cladding.

There is therefore also a need for a laser-to-fiber coupling arrangementthat reduces or eliminates coupling of focused radiant energy into thecladding of an optical fiber, rather than into the core.

SUMMARY OF THE INVENTION

It is accordingly a first objective of the invention to provide anapparatus and method for coupling a laser to an optical fiber or otherprimary optical transmission system, in which energy that does notcouple to the primary optical transmission system is diverted to preventdamage to components of the coupling connector, and which thereforeextends the useful life of the connector, ensures efficient coupling ofa portion of the radiant energy to the transmission system, enables theuse of smaller transmission components, for example fibers with smallerdiameters and/or acceptance angles, and that accommodates greatertolerances in aligning the transmission system to the radiant energysource.

It is a second objective of the invention to provide an improved methodof coupling a laser to an optical fiber in a way that facilitates use ofrelatively small optical fibers while minimizing damage to the couplingconnector and components thereof, and yet that is compatible withstandard laser coupling connectors, including medical laser industrystandard metal connectors.

It is a third objective of the invention to provide an arrangement forcoupling a laser to an optical fiber in a way that reduces or eliminatescoupling to the cladding rather than the core.

These objectives are achieved, in accordance with the principles of apreferred embodiment of the invention, by a connector for coupling aradiant energy source to a primary optical transmission system in whichthe connector includes a housing that supports a proximal end of thetransmission system, and in which the proximal end is surrounded by anoptical window which maintains an elongated secondary radiant energytransmission air path that terminates in a single faceted reflector, theproximal end, secondary radiant energy transmission air path, andreflector all being aligned on a common axis.

The secondary radiant energy path is arranged such that radiant energycoupled to the proximal end is transmitted by the primary optical systemwhile radiant energy that is not coupled to the proximal end or is losttherefrom is conducted along the secondary transmission path to thereflector and re-directed back through the secondary transmission path.

The re-directed energy is preferably diverted to a structure or locationwhere it can be safely dissipated, such as the radiant energy sourceitself. For example, in an enclosed laser system, once the radiantenergy travels outside the optical window and into the enclosed lasersystem, the radiant energy diverges so rapidly that the energy is lowenough to eliminate the need for a heat sink.

In an especially preferred embodiment of the invention, the reflectormay be a reflector member that includes a cylindrical body portionpositioned in a cylindrical housing with an axial bore therethrough forthe primary transmission system, with the reflector, window, andtransmission paths being supported and enclosed in axial spaced apartrelation to the cylindrical housing.

The reflector can be formed of metal such as copper, or can be made of ahigh temperature material such as ceramic with a deposited reflectivesurface or layer, the deposited surfaces being displaced from the outputend of the secondary transmission path far enough to reduce the energydensity of the radiant energy incident thereon to non-destructivelevels.

Preferably, the connector has dimensions and accommodates a coupling nutor other mechanism that permits the connector to be used in place of aconventional connector of the type illustrated in FIG. 2, without theneed to modify the apparatus in which it is to be used. For example, apreferred connector for use with the apparatus illustrated in FIG. 1should be capable of coupling directly to coupling connector 18 of FIG.1, in place of conventional connector 30, although it is of coursewithin the scope of the invention to require an adapter.

Further, in order to achieve the objective of reducing or eliminatingcoupling to the cladding of the fiber, which is not solved byre-directing errant radiant energy, the invention provides for removalof a section of cladding and the terminus of the fiber.

The method of the invention involves the steps of transferring radiantenergy from a focused source by directing a focused region of the sourceto a predetermined plane; conducting a portion of the focused radiantenergy incident on the plane along a first path; conducting theremaining portion of the focused radiant energy incident on the planealong a second path; reversing the direction of the remaining portion ata predetermined location, and directing the remaining portion backtoward the focused laser source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view, partly in cross-section, showing a conventionalradiant energy transmission system.

FIG. 2 is a cross-sectional view of a conventional fiber opticconnector.

FIG. 3 is a cross-sectional view of a fiber optic connector constructedin accordance with the principles of a preferred embodiment of theinvention.

FIG. 4 is a plan view, partly in cross-section, showing a radiant energytransmission system that utilizes the connector of FIG. 3.

FIG. 5 is a cross-sectional view of a rear portion of the connector ofFIG. 3, including a reflector and secondary optical transmission path.

FIG. 6 is a cross-sectional view of an alternative rear portion of theconnector of FIG. 3.

FIG. 7 is a cross-sectional view of a variation of the connectorstructure illustrated in FIG. 5.

FIG. 8 is a cross-sectional view of an arrangement for reducing oreliminating coupling to the cladding of an optical fiber, which may beused separately or in connection with any of the other embodiments ofthe invention.

FIG. 9 is a cross-sectional view of a modification of the arrangement ofFIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 illustrates a connector constructed in accordance with theprinciples of a preferred embodiment of the invention, for coupling to acoupling connector corresponding to coupling connector 18 illustrated inFIG. 1. FIG. 4 illustrates the operation of a connector of the typeillustrated in FIG. 3 upon coupling to a conventional apparatus of thetype illustrated in FIG. 1. It will of course be appreciated by thoseskilled in the art that the principles of the invention may be appliedto laser apparatus other than the apparatus illustrated in FIGS. 1 and4, and that the invention is not intended to be limited to a particularlaser source or connector coupling arrangement.

Turning to FIG. 3, connector 42 is identical in size to connector 30 ofFIG. 2, but differs in that it further includes a singlefaceted-reflector 36, a secondary transmission path 33, and an opticalwindow 39. The optical window 39 functions to ensure a clean surface onthe reflector 36 and prevents a plume from contaminating the condensinglens 20 as a result of accidental thermal ablation of the connector'smetal ferrule 31 or reflector 36, and may be formed of a variety oftransmissive materials such as quartz, sapphire, or diamond. The windowalso has a fiber clearance hole 41 corresponding in size and position tothe fiber clearance hole 38 shown in FIG. 2. However, after polishing ofthe fiber following insertion and adhesion or crimping of the fiber orfiber cable, both the distal end of the optical window 39 and the distalend of the optical fiber 35 are flush with the distal end of ferrule 31.

The reflector 36 is formed on a planar surface of the ferrule 31,surrounds the optical fiber, and is displaced far enough from the focalplane 16 to reduce the energy density of the radiant energy incident onthe surface to non-destructive levels, with the secondary transmissionpath 33 extending from the reflector 36 back through the window 39 so asto provide a path where the focused radiant energy can freely defocus,thereby further lowering the energy density. The reflector functions toreflect radiant energy that may miss the proximal end of the fiber dueto the fiber core being smaller than the focused spot, the NA of laserbeing greater than the NA of the fiber, and/or the due to misalignmentof the focused radiant energy relative to the connector coupler 18.Suitable materials for the reflector 36 include metals such as copper,or a high temperature substrate such as ceramic on which is formed areflective surface or layer.

As illustrated in FIG. 4, the radiant energy 17 d not captured by thefiber core is simply reflected back into the laser system enclosure,where it can safely dissipate, as described above. Alternatively, asillustrated in FIGS. 5–7, the reflector may be replaced by a heat sinkor a combination of both a reflector and a heat sink. For example, asillustrated in FIG. 5, the combination reflector and heat sink mayinclude a series of optical transmitting windows 45, each of whichreflects and absorbs part of the errant energy 17 d, thereby permittingthe invention to be used in applications where too much reflected lightwould otherwise travel into the laser and cause thermal damage or createoscillation problems in the lasing cavity. Because the energy absorbedby the attenuators could damage materials in the fiber optic cable 32, asleeve 47 may be included to ensure that the cable materials aresufficiently isolated from the heat sink.

A still further alternative is to replace the metal ferrule with a lighttransmissive ferrule 49 made of quartz, sapphire, or other lighttransmissive materials with no surrounding metal, as illustrated in FIG.6, which also shows a connector housing 51, and a quartz sleeve 55 towhich cable 32 is attached by an adhesive, the quartz sleeve beingisolated from ferrule 49 by an air space 57.

Finally, in a variation of the embodiment illustrated in FIG. 5, shownin FIG. 7, ferrule 31 may include a portion 58 that extends beyond thewindow 39, the window being recessed so as to expose the end of barefiber 34 and permit errant energy to expand and reduce its power densityto further minimize damage. In addition, recessing the window andexposing the fiber halps to keep the input surface free of debris. Theoptical window or attenuators may be treated, for example by roughening,so as to scatter or disperse incident errant radiant energy to furtherprovide a reduction in power density.

As is apparent from the above-description and accompanying drawings, themethod of the invention involves the steps of transferring radiantenergy from a focused source by directing a focused region of the sourceto a predetermined plane; conducting a portion of the focused radiantenergy incident on the plane along a first path; conducting theremaining portion of the focused radiant energy incident on the planealong a second path; reversing the direction of the remaining portion ata predetermined location, and directing the remaining portion backtoward the focused laser source.

As illustrated in FIG. 8, in order to solve the further problem ofcoupling to the cladding of the fiber 34, the invention provides forcomplete or partial removal of a section of cladding 62 along a section64 at the distal end of the core 66. This has the effect of setting thecladding back away from the focal plane of the laser light, or at leastof reducing the amount of cladding that can couple with the laser. Whencombined with a window 70 with roughening 68 and a reflector/heat sink72 as described above, coupling of focused radiant energy to thecladding 62 can be entirely eliminated.

In a variation of the cladding-coupling reduction or eliminationarrangement illustrated in FIG. 8, as shown in FIG. 9, the exposed fibercore section 64′ may be formed with an outward taper to enhance couplingto the core 66 rather than cladding 62.

Having thus described a preferred embodiment of the invention insufficient detail to enable those skilled in the art to make and use theinvention, it will nevertheless be appreciated that numerous variationsand modifications of the illustrated embodiment may be made withoutdeparting from the spirit of the invention, and it is intended that theinvention not be limited by the above description or accompanyingdrawings, but that it be defined solely in accordance with the appendedclaims.

1. Apparatus for coupling focused radiant energy from a laser to anoptical fiber having cladding, comprising: energy dissipating meansincluding a heat sink for dissipating energy that fails to couple to thefiber; and coupling minimizing means for reducing exposure of thecladding to the radiant energy, thereby minimizing coupling between theradiant energy and the cladding, wherein said coupling minimizing meansincludes a section of the optical fiber that has been partially strippedof cladding, wherein said partially-stripped section of the opticalfiber is situated at a distal end of the optical fiber, and wherein saidpartially-stripped section is surrounded by the heat sink, and whereinthe cladding of said partially stripped section is partially rather thancompletely removed.
 2. Apparatus as claimed in claim 1, wherein a distalend of said core is tapered to increase an area exposed to said focusedradiant energy.
 3. Apparatus as claimed in claim 1, wherein said fiberis surrounded by a quartz sleeve.